With large power LEDs being widely used in lighting and illumination, power-type LED driving circuits are increasingly important. Additionally, the luminous intensity of an LED, which is a current-type semiconductor light emitting device, is determined by the current flowing through the LED. Therefore, a power supply, which provides a constant current for driving LEDs, is in earnest demand.
Currently, two kinds of methods are commonly used for driving LEDs by means of electric mains (220 V or 110 V alternating-current supplies). One is to take advantage of RC voltage drop. In this case, the efficiency is so low that supplying electricity to an LED of 1 W will consume power of 4-6 W in the grid; and the power factor is extremely low, up to about 0.2, which not only causes heavy pollution to the grid, but also significantly reduces the lifetime of LEDs. The other is to employ a conventional AC/DC switch power supply with a constant voltage to supply electricity. In this case, the efficiency is about 70% and the power factor about 0.6. Due to its bulkiness, the brightness consistency of LEDs used in batches is poor. In addition, EMI (ElectroMagnetic Interference) is severe, and thereby causes heavy pollution to the grid.
To solve the problems described above, a HV9910 Universal High Brightness LED Driver is now commercially available, which has the circuit as shown in FIG. 1. It is designed to convert a high voltage source (AC85-265V after rectification, or DC8-450V) into a constant current source for supplying electricity to high brightness LEDs in series or series-parallel connections. HV9910 controls pulse width modulation (PWM) with the peak current having a constant frequency, which uses small inductors and external switches to minimize the loss of LED drivers. Unlike a conventional PWM controller, it employs a simple on/off control to adjust the LED current, thus simplifying the design of the controlling circuit.
As compared with a conventional LED driver, such a HV9910 driver has many advantages, such as simple design, lower cost, high efficiency (up to 93% or higher) and convenient control, etc. However, it employs pulse width modulation (PWM), which demands an accurate network compensation design for the sampled feedback signals obtained from power circuits. Parameters of such a compensation loop are affected by IC internal parameters, parameters of power circuits and layout and distribution parameters of printed circuit boards. Hence, as the operating frequency increases, such a design becomes increasingly difficult. It not only causes the increase of the costs for IC itself and peripheral components, but also causes the decrease of the stability of mass production. Additionally, the power factor of such a HV9910 driver is also extremely low. Only by introducing an inactive power correction circuit (as indicated by the circuit in a dashed box of FIG. 2) in circuits employing HV9910 when the input power of an LED driver does not exceed 25 W, can the power factor be improved. Even so, its power factor can only be raised up to 0.85.